We assume that the base AC 10 reflects proficiency in not getting hit. Armor then adds on top. Trying to force it into armor would just add a pointless bonus that we'd offset across the board. https://t.co/yJiwqaw0Mb

@HellcowKeithOut of curiosity, if this is the case, what’s the in-fiction/story thought process in the Warforged’s AC calculations? would know better, but i see it as warforged’s body physically improving

I agree. A warforged is a suit of magical living armor. Adding proficiency bonus to AC isn’t supposed to reflect greater skill at avoiding attacks, it reflects the armor and defensive enchantments of a warforged evolving and improving.